This invention relates generally to semiconductor devices and more particularly to high electron mobility field effect transistors, also known as modulation doped field effect transistors.
A relatively new heterostructure exhibiting great operational speed has appeared in the field of transistor technology. The device is currently known by several different names, including: high electron mobility transistor (HEMT), high electron mobility field effect transistor (HEMFET), modulation doped field effect transistors (MODFET), and two dimensional electron gas field effect transistor (TEGFET). The device is generally characterized by the combination of doped aluminum gallium arsenide (AlGaAs) on undoped gallium arsenide (GaAs) separated by a heterojunction. This causes a quasi two-dimensional electron gas (2DEG) to be produced and confined to the small band-gap side of the heterointerface. Charge carriers, i.e. electrons, are introduced into the structure by the higher band-gap material aluminum gallium arsenide, whereupon they then diffuse to the lower band-gap material gallium arsenide where they are confined by the energy barrier at the heterojunction. Since the gallium arsenide layer is not doped, very high electron mobility and large electron velocities at very small values is permitted. Control of the charge, i.e. electron flow, between two electrodes called the source and drain is provided by an intermediate Schottky barrier electrode, called a gate electrode, which is placed on the doped gallium arsenide layer or is recessed and placed on the doped aluminum gallium arsenide layer. This device has been described in much greater detail in an article entitled, "The HEMT: A Superfast Transistor", by H. Morkoc, et al. in the IEEE Spectrum, February, 1984, at pp. 28-35. This publication, moreover, is meant to be incorporated herein by reference.
It is an object of the present invention, therefore, to provide a new and improved semiconductor device.
It is another object of the invention to provide an improvement in field effect transistors.
It is yet another object of the invention to provide an improvement in ultra high speed field effect transistors.
Still a further object of the invention is to provide an improvement in high electron mobility field effect transistors.